Printing in register on sheets

Abstract

This invention is concerned with the high speed printing of a line of text on a sheet in very accurate registration with an edge of the sheet or another feature (for example an existing print line) on the sheet. The sheet is fed without preregistration on to a track (10, FIG. 1) which passes the printing station (20). A detector (16) adjacent the track senses the arrival of the edge or some other feature of the sheet and a printing control means (14) responsive to the detector signal initiates the firing of the printing hammer (22) when the print line reaches the printing station, a registered operation being thereby effected without stopping the sheet. The detector signal may start the operation of a counter (70, FIG. 4) and a firing signal is sent to the hammer control circuit (100, 102, 104) when the count reaches a value dependent on the distance between the detector and the point at which the printing operation takes place and (unless the new print is to be in alignment with a detected print line) also dependent on the distance on the sheet between the edge or feature detected and the position at which the new print is to appear.

We claim: 
     
       1. Apparatus for printing a line of characters in a desired print line location on each of a series of sheets so that the resulting printed line of each sheet is registered with respect to a feature of the sheet, comprising: means defining a flow path for the sheets;   a printing station located at a predetermined point along the flow path and comprising impact printing means including individually rotatably adjustable print wheels mounted on a common shaft, the axis of which is transverse and fixed with respect to said flow path, each print wheel having a plurality of character faces spaced around its periphery and individually selectable by rotation of each wheel about the said axis to predetermined print positions, the said axis extending in a direction parallel to the required line of characters on the sheet passing through the apparatus, and a hammer to cause impact printing of a sheet against a type face whereby the print wheels together print a line of characters, one for each wheel, in a single operation, each character printed being determined by the respective rotated position of the corresponding print wheel;   means for locking the print wheels in the positions to which they have been rotated;   sheet feeding means for feeding a succession of sheets the spacing between which may vary, continuously and at a substantially constant speed along the flow path and through the printing station,   sensing means detecting the arrival of the said feature of the sheet at a given point along the flow path and providing a corresponding electric signal; and   printing control means responsive to said signal from said sensing means and to the speed of travel of the sheet to initiate printing hammer firing when the desired print line location on the sheet reaches the printing station, whereby regardless of the speed of travel of, intervals between the individual sheets, or time of arrival of the sheets at the printing station, printing of a line of characters is effected by the stationary print wheels in register with respect to the said feature of the sheet without substantially changing the speed of the sheets.   
     
     
       2. Apparatus in accordance with claim 1, comprising a spring for the printing hammer, a cam for driving the hammer against the spring and which maintains the hammer against the spring until printing is initiated, and means for driving the cam to its hammer release position, allowing the spring to throw the hammer towards the print wheels; the apparatus further including a stop which is hit by the hammer when the hammer reaches the point of printing.   
     
     
       3. Apparatus in accordance with claim 1, in which the printing control means includes: a first counter, the operation of which is initiated by the signal from the detection means;   a circuit which initiates printer hammer firing when the counter reaches a predetermined count; and   a pulse generator supplying the counter with clock pulses at a rate corresponding to the speed of movement of the sheet along the flow path.   
     
     
       4. Apparatus in accordance with claim 3, in which the detecting means provides a first signal in response to the detection of the edge of a sheet and a second signal responsive to the detection of an existing mark on the sheet, the apparatus comprising: a second counter connected to receive the first and second signals; and   a comparator;   the first signal initiating counting in the second counter and the second signal applying the contents of the second counter, representing the distance between the said edge and the existing mark, to the comparator means to determine whether the said distance lies within a predetermined range; and wherein the second signal additionally initiates counting in the first counter.   
     
     
       5. Apparatus in accordance with claim 3 or 4, including: means for preloading the first counter with a value dependent on both the distance between the detector and the point along the flow path at which the printing operation takes place, and the distance on the sheet between the feature detected and the printing line location on the sheet; and   in which the first counter counts clock pulses, starting from the sensing of the said feature by the detecting means, and initiates printing hammer firing when the count reaches the preloaded value.   
     
     
       6. Apparatus in accordance with claim 3, in which the pulse generator includes a shaft encoder coupled to a shaft for driving a belt on which the sheet is advanced along the flow line to the printing station, for providing the said clock pulses.